We evaluated the activities of ceftriaxone (15 mg/kg), netilmicin (6 mg/kg), and their combination given intramuscularly once daily for 4 days for the treatment of experimental Escherichia coli endocarditis in rabbits. In vitro, a greater rate of killing and an increased trough serum bactericidal titer (P less than 0.01) were achieved with the combination. In vivo, the combination had a greater bactericidal effect (P less than 0.01) and resulted in a greater number of sterile vegetations (P less than 0.05) than single-drug therapy. Thus, in vivo, an increased effect can be obtained despite a single daily dose of a long-acting cephalosporin and an aminoglycoside.